The effect of the protein environment on the short-lived compound I state of cytochrome P450cam was analyzed in detail by decomposing the ONIOM(B3LYP:AMBER) interaction energy between the active site and the surrounding protein environment into electrostatic, van der Waals (vdW), and polarization terms. The electrostatic effect was the largest in magnitude, followed by the vdW effect, and then by the polarization effect. The contributions of individual residues to the environmental effect were also evaluated.